(a) Field of the invention:
The present invention relates to a varifocal lens system capable of varying focal length of said lens system as a whole by inserting and removing an auxiliary lens group into and out of the optical path of a photographing lens system.
(b) Description of the prior art:
Recently, it is strongly demanded, even for a popular camera designed as an integral unit of a camera body and a photographing lens system, to photograph at wide-position and tele-position, etc. by varying focal length of the photographing lens system. In order to satisfy this demand, it has been proposed to design a photographing lens system for a popular camera as a zoom lens system capable of varying focal length thereof or as a lens system capable of varying focal length by inserting and removing an auxiliary lens into and out of the optical path of a photographing lens system.
As a lens system of the latter type, it has already been known the one disclosed by Japanese published unexamined patent application No. 108510/83. This lens system comprises a front lens group having positive refractive power and a rear lens group having negative refractive power, and designed to vary its focal length by shifting said front lens group toward the object side and inserting an auxiliary lens group having negative refractive power into the airspace formed by shifting said front lens group. However, this lens system has a defect that it requires a long time for varying focal length since said auxiliary lens group can be inserted only after said front lens group has been shifted on the extreme object side.
Further, said lens system requires an undesirably complicated and large mechanism for preventing the front lens group shifting operation from interfering with the auxiliary lens group inserting operation since the cam groove used for shifting the front lens group and the opening for inserting the auxiliary lens group are located nearly at the same location in the lens barrel.
Moreover, this varifocal lens system is set in a long focal length condition when said auxiliary lens group is inserted since it is designed to vary focal length thereof by inserting said auxiliary lens group having negative refractive power. As a result, the paraxial ray is set higher when said auxiliary lens group is inserted than that in the short focal length condition at the same numerical aperture. Accordingly, the paraxial ray incident on said auxiliary lens group is also high, thereby making it difficult to design said auxiliary lens group compact.